Process for shielding an electric or electronic circuit and shielding cap

ABSTRACT

A process is disclosed for screening a part of an electrical or electronic circuit, which part is surrounded by an electrically-conductive frame, from absorbing and/or emitting electromagnetic interference fields. The process comprises taking an electrically-conductive screening cap, with an open side bordered by a rim that contains a seating surface, and forming an elastic and electrically-conductive screening seal by directly applying the sealing compound to the seating surface in such a way that the screening seal projects beyond the seating surface. The screening cap is then placed with the screening seal on the electrically-conductive frame to fix the screening cap on the electrically-conductive frame.

TECHNICAL FIELD

The present invention concerns the field of electromagnetic screening.It deals with a process to screen an electrical or electronic circuitfrom absorbing and/or emitting disturbing electromagnetic rays wherebyan electrically-conductive screening cover has a seating surface with anelastic and electrically-conductive screening seal that projects beyondthe seating surface, and then the seating surface with the screeningseal is placed on and fixed to an electrically-conductive frame thatsurrounds the circuit.

The invention also concerns a screening cover to screen an electrical orelectronic circuit from absorbing and/or emitting disturbingelectromagnetic rays. The screening cover has a seating surface forplacing the screening cover on an electrically-conductive framesurrounding the circuit. The seating surface is provided with an elasticand electrically-conductive screening seal that projects beyond theseating surface.

Such a process and screening cover are e.g. prior art in German utilitymodel G66071805.

STATE OF THE ART

To electromagnetically screen electronic circuits or circuit parts sothat they do not absorb and/or emit especially high-frequency disturbingelectromagnetic interference fields, the circuits or circuit parts arecovered with an electrically-conductive cup or tub-shaped screeningcover. The edge or seating surface of the screening cover is placed onan electrically-conductive frame that surrounds the circuit, and it isfixed in place. The frame can e.g. consist of a copper layer that is onthe top of the p.c.b. which bears the circuit.

A problem with screening covers is getting a consistent electricalcontact that extends along the entire edge of the cover between thescreening cover and frame; another problem is securely fixing screeningcap in the screening position. To adapt the seating surface of thescreening cover to unevenness in the frame and prevent undesirable holesin the screen, the edge of the screening cover can e.g. be elastic orcontact the frame via a contact spring strip. It is also suggested inthe initially-cited document that the transition between the seatingsurface and frame be an electrically-conductive, elastically-deformablescreen seal. If the screen comprises a lid, the screening seal consistsof a flat foam layer on the bottom of the lid that either is madeelectrically-conductive from conductive additives (carbon or metalpowder), or by vapor-deposition of metals, or by adhering a metal film.If the screen comprises a screening cover, there is a groove in the edgeof the cover in which a peripheral ring of foam is inserted that iseither electrically-conductive itself, or has been madeelectrically-conductive by treating its surface. The groove is formed byspot-welding shaped sheet metal on the inside of the high edge of thescreening cover. The addition of such a groove not only adds more effortto the manufacture of the screening cover, it also takes up more space.In addition, the groove is difficult to manufacture when the shape ofthe screening cover is complicated and has angles.

PRESENTATION OF THE INVENTION

The problem of the invention is therefore to present a process to screenan electrical or electronic circuit and to present a screening coverthat is distinguished by outstanding screening, ease of use, and a greatdeal of flexibility.

The problem is solved with a process of the initially-cited type in thatthe screening seal is created by directly applying a sealing compound tothe seating surface. By directly applying the sealing compound accordingto the invention, the creation of a groove or another special recess forthe seal becomes superfluous. This is particularly advantageous when theseating surface of the screening cover is a simple edge. The sealingcompound is directly applied to the face of the edge following the(possibly complicated) path of the edge, and then anchored there. Aminimum of space and (expensive) sealing compound are used to create andoptimum screening seal for the screening cover.

A first preferred embodiment of the procedure according to the inventionis characterized in that the sealing compound is applied by anapplication process as disclosed in another context, i.e., for screeninghousings e.g. in DE C2 43 19 965. The pasty or slightly liquid sealingcompound can be applied in a precise dose and with a presetcross-section directly to the edge of the screening cover. Withcorresponding control (programming) of a dispenser that can move alongthree axes, complicated edge shapes of the screening cover can befollowed. The properties of the sealing compound are suitably adjustedso that the bead of sealing compound that is applied to the edge hardenson the edge without running. In a development of this embodiment,several sealing beads are applied on each other to form the screeningseal. The greater height of the screening seal compensates forcorrespondingly greater tolerances between the frame and contactsurface.

In a second preferred embodiment of the process according to theinvention, the sealing compound is applied to the seating surface byusing a flowable sealing compound. The screening cover is immersed withthe seating surface in a bath consisting of the sealing compound, andthen it is removed. The sealing compound wets the edge or seatingsurface of the screening cover and forms a bead that surrounds the edgeby means of surface tension when it is removed, and a peripheralscreening seal is formed after hardening. The advantage of thisembodiment is that the entire edge can be quickly and easily providedwith the screening seal without using a numerically-controlledapplicator and meter. The thickness of the forming screening seal islargely determined by the properties of the sealing compound in the bathand can e.g. also be influenced by repeating the immersion procedure. Ifthe sealing compound is a (curing) material that hardens when exposed tohumidity, the immersion process should be carried out under an inertgas.

It is preferable when the sealing compound is an electrically-conductiveelastomer, especially based on silicone or polyethylene that containselectrically-conductive fillers. The screening seal is created bydirectly applying the sealing compound without additional steps.Examples of such compounds are found in European patent application 0643 551 and 0 643 552. Alternately, the sealing compound can be anelectrically-insulating elastomer, and the sealing compound is providedwith an electrically-conductive coating, especially in by metallization,after it is applied to the seating surface. This type of manufacture isparticularly advantageous when a screening cover is used that consistsof an electrically-insulating material, and when the screening cover andsealing compound are both provided with an electrically-conductivecoating, especially by metallization after the sealing compound has beenapplied to the seating surface. The expensive, electrically-conductiveand somewhat difficult to process sealing compound can be dispensedwith, and plastic screening covers can be used that are easy to make indifferent configurations with the usual methods of plastic processing(injection molding, blow-molding, etc.).

The screening cover can be affixed in various ways in the context of theinvention. In a preferred exemplary embodiment, the screening cover canbe mechanically fixed after it is placed on the frame. Auxiliary meanscan be used to mechanically affix it that press the screening cover withthe screening seal on the frame. The auxiliary means are preferably aspring clip, screw, an opposing housing part or a spring sheet metalpart that is attached to the screening cover, and especially a part ofthe screening cover made of sheet metal.

In another preferred exemplary embodiment, the screening cover adheresto the frame after the screening seal is placed on the frame. This canbe accomplished by using an electrically-conductive elastomer as thesealing compound that is adhesive or wet and can be hardened by heat orradiation; it is hardened after the screening cover is placed on theframe. An electrically-conductive elastomer can be used that is adhesiveor wet even after hardening. The screening cover can adhere to the frameby using an elastically-conductive thermoplastic or anelectrically-conductive hot-melt adhesive as the sealing compound, andthe screening cover is glued to the frame by increasing the temperatureafter it is placed on the frame.

The screening cover according to the invention is characterized in thatthe screening seal consisting of the sealing compound is molded onto theseating surface.

In a preferred embodiment, the screening cover according to theinvention is characterized in that the seating surface is an edge, andeither the screening seal is placed on the face of the edge, or thescreening seal encloses the edge.

Additional embodiments are found in the dependant claims.

SHORT EXPLANATION OF THE FIGURES

The invention will be further explained using exemplary embodiments inconnection with the drawing.

FIG. 1 A top view (FIG. 1a) of an example of a circuit on a p.c.b.without a screening cover, and a side view of a screening cover drawn indashed lines (FIG. 1b);

FIG. 2 A perspective view of the screening cover from FIG. 1b (with abottom opening);

FIG. 3 A screening seal placed on the screening cover from FIG. 2according to a first preferred exemplary embodiment of the procedureaccording to the invention. The screening cover is first positioned withthe opening (edge) facing upward (FIG. 3a), and then the screening sealis applied to the edge with a moving dispenser (FIG. 3b);

FIG. 4-6 Side view of various ways of attaching the screening coverwithin the framework of the invention, i.e., with an adhesive screeningseal (FIG. 4), a spring clip and/or a screw (FIG. 5), and by abutting anopposing housing part, etc. (FIG. 6);

FIGS. 7a-c Different steps in creating the screening seal by immersingthe screening cover in a bath consisting of a (liquid) sealing compoundaccording to second preferred exemplary embodiment of the invention;

FIGS. 8-10 Sections of different types of screening seals according tothe invention that have been made by immersion according to FIG. 7 (FIG.8), or by applying a second bead of sealing compound according to FIG. 3(FIG. 9), or by metallizing a non-conductive seal (FIG. 10);

FIG. 11 The application of a screening seal comparable to FIG. 3b on ascreening cover that is equipped with spring sheet metal parts forattachment; and

FIG. 12 Side view of attaching the screening cover from FIG. 11 withspring sheet metal parts that are inserted through openings into thep.c.b. and then elastically lock.

WAYS TO IMPLEMENT THE INVENTION

FIG. 1 shows a top view (FIG. 1a) and side view (FIG. 1b) of an exampleof a circuit on a p.c.b. without a screening cover. The circuit 10comprises a p.c.b. 11 (printed circuit or multilayer) on which severalelectronic components 13 (resistors, capacitors, diodes, ICs, etc.) arelocated and connected in a central area. The components 13 form apartial circuit that is to be screened from other partial circuits orthe environment so that no disturbing electromagnetic fields are emittedby the partial circuit or are absorbed from the outside. The partialcircuit is surrounded by a conductive frame 12 in the form of a topmetal layer on the p.c.b. 11. Of course, the partial circuit, frame 12and construction of the circuit 10 can have different shapes or designs.

The partial circuit with the components 13 is screened by a cup-shapedscreening cover (14) (drawn with dashed lines in FIG. 1b) and is shownin a perspective view in FIG. 2. The screening cover 14 is in the shapeof a rectangular box in the portrayed example. Of course, the screeningcover can be round or obliquely angled and have bulges, projections,etc. The screening cover 14 can be bent from a sheet, soldered or formed(deep-drawn). It can also consist of metallized plastic. The screeningcover 14 is placed over the partial circuit as indicated in FIG. 1b. Theedge neighboring the open side of the screening cover 14 forms theseating surface and lies on the frame 12 to create a tight andelectrically-conductive transition from the screening cover 14 to theframe 12.

To compensate for unevenness, an elastic and electrically-conductivescreening seal is provided between the edge or seating surface of thescreening cover 14 and the frame 12. The screening seal provides theelectrical contact between the screening cover 14 and the frame 12 andcan compensate for tolerances of several tenths of a millimeter. In onepreferred exemplary embodiment of the invention, the screening seal iscreated and applied to the screening cover 14 in that the sealingcompound that forms the screening seal is directly applied to the edge(dispensed) in the form a bead. In FIG. 3a, the screening cover 14 withthe edge seating surface 15 is placed facing up on the base of adispensing device. In FIG. 3b, the dispenser 17 as e.g. described in DEC2 43 19 965 travels by numerical control along the seating surface 15.The sealing compound 21 (preferably pasty or thixotropic) is depositedunder pressure from a nozzle 25 in the form of a bead onto the seatingsurface 15, and it forms a screening seal 16. If the screening cover 14has conventional wall thickness (sheet metal thickness) of a few tenthsof a millimeter, the thickness of the (usually round) bead is ca 0.1 to2 mm.

The properties of the sealing compound 21 are adjusted so that the beadbasically retains its shape and cross-section after being placed on theseating surface 15, and it also adheres to the seating surface. Anelectrically-conductive elastomer based on silicone or polyurethane thatcontains electrically-conductive fillers is used as the sealing compound21, or an electrically-conductive elastomer is used. In this case, thesealing compound 21 is provided with an electrically-conductive coat,especially in the form of a metal coating 24 after being applied to theseating surface 15.

The sealing compound 21 can have a solid, plastic or adhesiveconsistency. It can be directly curable or hardenable by humidity whenthe screening cover 14 is not to be adhered tightly to the frame 12. Itcan also curable by heat or radiation (UV, IR, X-ray or laser) when e.g.the screening cover 14 is to be tightly adhered with the screening sealto the frame 12. The compound is only hardened in this case when thescreening cover 14 has been put in place. If a sealing compound is usedthat is adhesive after it has hardened, the screening cover can also beadhered after it is hardened. Alternately, it is also conceivable forthe sealing compound to be an electrically-conductive thermoplastic oran electrically-conductive hot-melt adhesive, and the screening cover 14is adhered to the frame 12 by increasing the temperature when it is onthe frame 12. Likewise, electrically-conductive or isotropic plasticscan be used as the sealing compound 21.

The properties of the sealing compound applied in FIG. 3b allow thescreening cover 14 to be directly applied to the frame. The screeningcover 14 with the screening seal 16 can be directly placed on the frame12 and adhered tight (FIG. 4). If the adhesive force is sufficient forpermanently fixing the screening cover 14, additional auxiliary agentscan be dispensed with which makes use easier. If the adhesive force isinsufficient or if the screening seal is not adhesive, additional(primarily mechanical) auxiliary means are used to fix the screeningcover 14. In the example in FIG. 5, a spring clip 18 is used whose endsare attached to the p.c.b., and whose elastic middle part is pressedfrom above onto the screening cover 14. Instead of a spring clip 18,clamps or similar fasteners can be used. Furthermore, auxiliary meanssuch as one or more screws 26 can be used instead of or in addition tothe spring clip 18. The screws are inserted through a hole in thescreening cover 14 and screwed to the p.c.b. 11, or inserted through ahole in the p.c.b. 11 and screwed with an insert to the screening cover14. Another option for fixing the cover is shown in FIG. 6. An opposinghousing part 19 etc. is used to press the screening cover 14 against thep.c.b. 11 which abuts a base 20.

Another easy and simple method for fixing the cover is shown in FIGS. 11and 12 in conjunction with a screening cover made of sheet metal. Ascreening cover 14 is used that has at least one and preferably two ormore opposing spring sheet metal parts 27, 28. The strips of springsheet metal 27, 28 extend upward from the seating surface 15 and arecurved like a hook. They can be a part of the sheet (as shown in FIG.11) from which the screening cover 14 is made. They can also be made ofseparate strips of sheet metal and be spot-welded, riveted, glued orsoldered to the screening cover 14. In these cases, it is useful to bendthe spring sheet metal parts 27, 38 slightly outward so that the seatingsurface 14 is completely covered with the screening seal 16.

The screening cover with the spring sheet metal parts 27, 28 is providedwith the screening seal 16 as shown in FIG. 3b. Then the screening cover14 from FIG. 12 can be mechanically affixed to the p.c.b. 11 with thespring sheet metal parts 27, 28. In the p.c.b. 11 are through-holes 29,30 (drawn with dashed line in FIG. 12) at suitable sites, e.g. in theform of elongated holes or slots through which the spring sheet metalparts 27, 28 can be inserted and elastically lock when the screeningcover 14 is put in place. This allows the screening cover 14 to beattached in a particularly easy and secure manner, and it is suitablefor automated manufacture and does not require many additional parts andinstallation effort. In addition, a screening cover 14 affixed in thismanner is easy to service and repair and can be removed and reused bypressing together the sheet-metal parts on the bottom of the p.c.b. 11and pressing them back through the openings 29, 30.

The process shown in FIG. 3 for applying the screening seal 16 with adispenser 17 requires corresponding equipment. In addition, thenumerical control of the dispenser 17 must be adjusted in a program tothe edge path of the respective screen cover 14. The amount of equipmentcan be clearly reduced when another process (immersion) is used to applythe screening seal to the seating surface 15 of the screening cover 14that e.g. is schematically represented in FIG. 7. With the immersionprocess, the screening cover 14 with the opening or seating surface 15facing down (FIG. 7a) is dipped in a bath with a liquid or highlyviscous sealing compound 21 (FIG. 7b). The sealing compound 21 in thecontainer 22 has a very even surface. If the edge of the screening cover14 is slowly and evenly dipped in the sealing compound 21 a few tenthsof a millimeter, the sealing compound 21 wets the edge. When the sealingcover 14 is removed from the bath, the surface tension at the edge ofthe cover forms an annular bead of sealant that forms a screening seal16 which is tightly bonded to the edge after it hardens (FIG. 7c; seesection in FIG. 8).

It is hence very easy to cover in a single step the entire edge of thescreening cover 14 with a very even screening seal. The properties ofthe sealing compound (when liquid and uncured) must be adjusted so thatthe wetting behavior and surface tension produce the desired results. Itis useful for the seating surface 15 of the screening cover 14 to be ina single plane for the immersion procedure. If the seating surface 15 isin different planes, the dispensing process in FIG. 3 is useful in whichthe dispenser 17 can be moved in three axes.

The dispensing process in FIG. 3 primarily produces a screening seal 16with a round cross-section, while the dipping process in FIG. 7 tends toform a drop-shaped cross-section according to FIG. 8. If the screeningseal 16 is to compensate for greater tolerances, its height must begreater. This can be accomplished by increasing the diameter of thescreening seal. This however requires a comparatively large amount ofexpensive sealant. It is better to place several sealing beads 16 a,b oneach other as in FIG. 9 using the process in FIG. 3 that then form anarrow, high screening seal 16.

Another option for forming a conductive screening seal 16 is shown inFIG. 10. The screening seal 16 comprises a seal core 23 consisting of anelectrically-insulating elastomer that is coated on the outside withmetal 24. The sealing core 23 can be applied as in FIG. 3, and it canalso be created by dipping as in FIG. 7. The metallization can becarried out in a manner known per se. A particularly advantageoussolution is to use a screening cover 14 consisting of an electricallyinsulating material. In this case, the screening cover 14 and thesealing compound or sealing core 23 are both provided with anelectrically-conductive coating, especially in the form of metal coating24 after applying the sealing compound to the seating surface 15.

Overall, the invention presents a screening process, or a screeningcover that is easy to implement and flexible to use, and that isdistinguished by a low amount of manufacturing effort while retaining ahigh screening quality.

REFERENCE NUMBER LIST 10 Circuit 11 P.c.b. 12 Frame (conductive) 13Component (electronic) 14 Screening cover 15 Seating surface (edge) 16Screening seal 16a, b Sealant bead 17 Dispenser 18 Spring clip 19Housing part 20 Base 21 Sealing compound 22 Container 23 Sealing core 24Metal coating 25 Nozzle 26 Screw 27, 28 Spring sheet metal part 29, 30Opening

What is claimed is:
 1. A process for screening a part of an electricalor electronic circuit which part is surrounded by anelectrically-conductive frame, from absorbing and/or emittingelectromagnetic interference fields, said process comprising the stepsof: taking an electrically-conductive screening cap the open side ofwhich is bordered by a rim, said rim being provided with a seatingsurface; forming an elastic and electrically-conductive screening sealby using a flowable and elastically hardening sealing compound, dippingsaid screening cap with said rim in a bath of said sealing compound suchthat said sealing compound adheres to said seating surface; and removingsaid screening cap with said adhering sealing compound from said bath,to apply said sealing compound on said seating surface such that saidscreening seal projects beyond the seating surface; placing saidscreening cap with said screening seal on said electrically-conductiveframe; and fixing said screening cap on said electrically-conductiveframe.
 2. A screening cap for screening a part of an electrical orelectronic circuit from absorbing and/or emitting electromagneticinterference fields, said electrical or electronic circuit beingarranged on a circuit board, and said part of said electrical orelectronic circuit being surrounded by an electrically-conductive frameon said circuit board, whereby said screening cap comprises side wallswhich run essentially vertical with respect to said circuit board andwhich are bordered by seating surfaces adapted to make contact with saidelectrically-conductive frame on said circuit board when said screeningcap is placed on said circuit board, and whereby said seating surfacesare provided with an elastic and electrically-conductive screening sealthat adheres to and projects beyond said seating surfaces.
 3. Ascreening cap according to claim 2, characterized in that the seatingsurfaces are edges, and the screening seal is placed on the face of theedges.
 4. A screening cap according to claim 2, characterized in thatthe seating surfaces are edges, and the screening seal surrounds theedges.
 5. A screening cap according to one of claims 3, 4, or 2,characterized in that the screening seal has the shape of one or moresealing beads.
 6. A screening cap according to one of claims 3, 4, or 2,characterized in that the screening seal consists of anelectrically-conductive elastomer, especially based on silicone orpolyurethane with electrically-conductive fillers.
 7. A screening coveraccording to claim 6, characterized in that the electrically-conductiveelastomer has an adhesive or wet consistency and can be hardened by heator radiation.
 8. A screening cap according to one of claims 3, 4, or 2,characterized in that the screening seal consists of anelectrically-conductive thermoplastic or an electrically-conductivehot-melt adhesive.
 9. A screening cap according to one of claims 3, 4,or 2, characterized in that the screening seal comprises a sealing coreconsisting of an electrically-insulating elastomer that is provided witha metal coating.
 10. A screening cap according to one of claims 3, 4, or2, characterized in that the screening cap consists of a metallizedinsulation.
 11. A process for screening a part of an electrical orelectronic circuit from absorbing and/or emitting electromagneticinterference fields, whereby said electrical or electronic circuit isarranged on a circuit board, and whereby said part of said electrical orelectronic circuit is surrounded by an electrically-conductive frame onsaid circuit board, said process comprising the steps of: taking anelectrically-conductive screening cap with side walls which runessentially vertical with respect to said circuit board and which arebordered by seating surfaces adapted to make contact with saidelectrically-conductive frame on said circuit board when said screeningcap is placed on said circuit board; forming an elastic andelectrically-conductive screening seal by directly applying anelectrically-conductive sealing compound on said seating surfaces ofsaid side walls such that said electrically-conductive screening sealadheres to and projects beyond said seating surfaces; placing saidscreening cap with said electrically conductive screening seal on saidelectrically-conductive frame; and fixing said screening cap to saidcircuit board.
 12. A process according to claim 11, wherein saidadhesive sealing compound is applied to said seating surfaces by: usinga flowable and elastically hardening sealing compound; dipping saidscreening cap with said seating surfaces in a bath of said sealingcompound such that said sealing compound adheres to said seatingsurfaces; and removing said screening cap with said adhering sealingcompound from said bath.
 13. A process according to claim 11,characterized in that the sealing compound is applied in the form of atleast one bead to the seating surfaces by a dispenser through a nozzle.14. A process according to claim 13, characterized in that severalsealing beads are applied on top of each other to from the screeningseal.
 15. A process according to one of the claims 2, 3, 11, or 12,characterized in that an electrically-conductive elastomer is used asthe sealing compound.
 16. A process according to claim 15, characterizedin that an electrically-conductive elastomer based on silicone orpolyurethane is used that contains electrically-conductive fillers. 17.A process according to one of claims 2, 3, 11, or 12, characterized inthat an electrically-insulating elastomer is used as the sealingcompound, and the sealing compound is provided with anelectrically-conductive coating, especially in the form of metal coatingafter it is applied to the seating surfaces.
 18. A process according toclaim 17, characterized in that a screening cap is used that consists ofan electrically-insulating material, and the screening cap and sealingcompound are both provided with an electrically-conductive coating,especially in the form of metal cover, after the sealing compound hasbeen applied to the seating surfaces.
 19. A process according to one ofthe claims 2, 3, 11, or 12, characterized in thatelectrically-conductive or isotropic plastics are used as the sealingcompound.
 20. A process according to one of the claims 11 or 12,characterized in that the screening cap is mechanically fixed after itis placed on the frame.
 21. A process according to claim 20,characterized in that auxiliary means are used to mechanically affix thecover, and the auxiliary means press the screening cap with thescreening seal against the frame.
 22. A process according to claim 21,characterized in that the auxiliary means comprise a spring clip.
 23. Aprocess according to claim 21, characterized in that the auxiliary meanscomprise a screw.
 24. A process according to claim 21, characterized inthat the auxiliary means comprise an opposing housing part.
 25. Aprocess according to claim 21, characterized in that the auxiliary meanscomprise spring sheet metal parts that are on the screening cover.
 26. Aprocess according to one of the claims 11 or 12, characterized in thatthe screening cap is adhered to the frame with the screening seal afterit is placed on the frame.
 27. A process according to claim 26,characterized in that an adhesive or wet electrically-conductiveelastomer is used as the sealing compound.
 28. A process according toclaim 27, characterized in that the sealing compound is hardenable byheat or radiation, and it is hardened after the screening cap is placedon the frame.
 29. A process according to claim 26, characterized in thatan electrically-conductive thermoplastic or hot-melt adhesive is used asthe sealing compound, and the screening cap is adhered after it isplaced on the frame by increasing the temperature at the frame.